For existing mobile radio standards, a multitude of different frequency ranges have been defined or allocated worldwide by regulating authorities. Thus, for a possibly worldwide and unlimited usability of a mobile radio phone, the manufacturer is faced with the challenge to cover these different frequency bands.
In modern applications, increasingly integrated complex high-frequency circuits are used, which allow to save more and more of the discrete devices necessitated earlier on the printed circuit board. These high-frequency chips include entire receivers, transmitters, frequency generation, signal processing and possibly supply voltage regulation.
However, so far, it is still not possible to integrate, for example, very selective high-quality filters in the high-frequency range which are, however, necessitated in the high-frequency signal path due to the way the system operates. Thereby, every frequency band to be supported will be treated separately and the paths will be combined directly in front of the antenna by means of a high-frequency signal multiplexer.
Since all signal paths to be supported are treated differently on the printed circuit board, the chip manufacturer is faced with the task to either provide a respective number of signal sources (transmitter outputs) and signal sinks (receiver inputs) on the high-frequency chip, or alternatively to offer a lower number of all-purpose sources and sinks, which can, however, then be connected to the individual transmit and receive paths by means of further external switches. Here, the chip manufacturer will rather favor the second method since this consumes less chip area and circuit effort and is generally the less expensive alternative.
The telephone manufacturer, however, will rather try to use the first alternative, i.e., to favor arrangements that are as simple as possible with few switches on the printed circuit board. This saves device costs, development effort and usually allows a better quality of the signal chains, since the signal loss within the chain becomes smaller due to the missing switches.
A switch within the signal path naturally causes signal losses caused by the finite conductance and the parasitic capacitances. Thereby, external switches can normally be built with better performance by the use of specific technologies than those produced within a chip by semiconductor technology.